As opposed to sit-ups, which target only your abdominal muscles, planks recruit several groups of muscles along your sides, front, and back. If you want a strong core — especially the kind that would give you 6-pack-like definition across your abs — you need to challenge all of these muscles.

"Sit-ups or crunches strengthen just a few muscle groups," write the authors of the Harvard Healthbeat newsletter, which summarizes the report's key takeaways. "Through dynamic patterns of movement, a good core workout helps strengthen the entire set of core muscles you use every day."

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Conventional crunches may also be hard on different parts of your body, including your back, which gets pushed against the floor. Additionally, when you pull your body up into a sit-up position, you're working a group of muscles called the hip flexors which run from your thighs to the lumbar vertebrae in your lower back. When these muscles get too tight or become overly-strengthened, . This can cause pain or discomfort in your lower back.

And unlike ab-crunch machines, planks don't require a single piece of equipment, so you can do them anywhere.

Ready to give them a try?

The folks at Health.com recommend starting out by lying face-down with your legs extended and your elbows bent, directly under your shoulders, with your hands clasped. Your feet should be hip-width apart; your elbows should be shoulder-width apart. Next, tighten your abs and tuck your toes to elevate your body, keeping your forearms on the ground. Your body should be in a straight line from your head to your heels. To start, hold it for one minute.

As you get stronger, you can gradually build up to maintain the position for longer periods of time, and extend your arms to hold yourself up on your palms.

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